Transmitter – Rainbow Electronics MAX7032 User Manual

Page 16

MAX7032

Low-Cost, Crystal-Based, Programmable,
ASK/FSK Transceiver with Fractional-N PLL

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The maximum and minimum peak detectors can be
used together to form a data slicer threshold voltage at
a value midway between the maximum and minimum
voltage levels of the data stream (see the Data Slicer
section and Figure 4). The RC time constant of the
peak-detector combining network should be set to at
least 5 times the data period.

If there is an event that causes a significant change in
the magnitude of the baseband signal, such as an AGC
gain switch or a power-up transient, the peak detectors
may “catch” a false level. If a false peak is detected,
the slicing level is incorrect. The MAX7032 has a fea-
ture called peak-detector track enable (TRK_EN),
where the peak-detector outputs can be reset (see
Figure 5). If TRK_EN is set (logic 1), both the maximum
and minimum peak detectors follow the input signal.
When TRK_EN is cleared (logic 0), the peak detectors
revert to their normal operating mode. The TRK_EN
function is automatically enabled for a short time when-
ever the IC is first powered up, or transitions from trans-
mit to receive mode, or recovers from the sleep portion
of DRX mode, or when an AGC gain switch occurs
regardless of the bit setting. Since the peak detectors
exhibit a fast-attack/slow-decay response, this feature
allows for an extremely fast startup or AGC recovery.
See Figure 6 for an illustration of a fast-recovery
sequence. In addition to the automatic control of this
function, the TRK_EN bits can be controlled through the
serial interface (see the Serial Control Interface section).

Transmitter

Power Amplifier (PA)

The PA of the MAX7032 is a high-efficiency, open-
drain, Class C amplifier. The PA with proper output-

matching network can drive a wide range of antenna
impedances, which includes a small-loop PC board
trace and a 50

Ω antenna. The output-matching network

for a 50

Ω antenna is shown in the Typical Application

Circuit. The output-matching network suppresses the
carrier harmonics and transforms the antenna imped-
ance to an optimal impedance at PAOUT (pin 5). The
optimal impedance at PAOUT is 250

Ω.

When the output-matching network is properly tuned, the
PA transmits power with a high overall efficiency of up to
32%. The efficiency of the PA itself is more than 46%.
The output power is set by an external resistor at
PAOUT, and is also dependent on the external antenna
and antenna-matching network at the PA output.

MAX7032

PDMIN

TO SLICER

INPUT

BASEBAND

FILTER

MINIMUM PEAK

DETECTOR

MAXIMUM PEAK

DETECTOR

PDMAX

TRK_EN = 1

Figure 5. Peak-Detector Track Enable

Figure 6. Fast Receiver Recovery in FSK Mode Utilizing Peak
Detectors

200mV/div

DATA OUTPUT

2V/div

MIN PEAK DETECTOR

MAX PEAK DETECTOR

RECEIVER ENABLED, TRK_EN SET

TRK_EN CLEARED

FILTER OUTPUT

DATA OUTPUT

100

µs/div

MAX7032

PDMAX

PDMIN

DATA

SLICER

DATA

PEAK

DET

PEAK

DET

Figure 4. Generating Data Slicer Threshold Using the Peak
Detectors